The
shielded loop antenna was first used in WW2 for direction finding
utilizing its extremely sharp null. The depth of the null
depends on a number of factors, one of them is symetry of the active
element, the Q of the resonant system and spacing of the loop with
respect to other conducting surfaces. It has no real lower
frequency limit, save that of the Q of the system given the large
number of loop turns required to achieve resonance. Loop
antennas also featured in early crystal set designs, however, they
mostly failed to notice the value of resonance in the loop.

There
is essentially only one differance between the shielded and unshielded
loop as a reception antenna. The unsheilded loop will pick up
electromagetic radiation, that is true EM waves (photons) and
also responded to electrostatically induced noise. This noise in the
average industrial of suburban envoiroment will typically come from
flourescent lights, brush motors, corona discharge from HV power lines,
and the radiated rubbish emitted by the line scan of TV sets.
Increasingly the plasma screen TVs radiate broad
spectrum garbage that has all but destroyed short wave reception.
The shielded loop , on the other hand, is allmost
insensitive to electrostatically induced noise. Nearly all of the
energy content of long wave and medium wave interference locally
generated is electrostatic. Very little of that noise energy gets
transformed to real EM radiation (photons).

The
sheilded loop is capable of remarkable feats of long or medium wave
reception, on paths that "should not exist", in envoiroments that are
beset by electrical noise. They have moderate directivity, but
their true merit lies in their nulls which on practical antennas can
give 60db attenuation to an unwanted transmitter. It is even
possible to use the shielded loop antenna to completely null out the
groundwave radiation on a local medium wave broadcast station and
listen to it via its ionospheric echo during the day. It sounds
exactly like a short wave broadcast!

Radio hams are only dimmly
aware of this remarkable antenna for working 80 and 160meter bands.
many who try them superficially, discard them in favour of long
wire antennas. The reason is that the loop is an extremely inefficient
and low gain antenna. A long wire allways picks up more signal, but it
cannot discriminate against the noise. The true merit of the loop
is its profound ability to reject noise, and nothing else. If
your station is in a quiet rural site, then the loop gives you nothing
that you dont allready have. ( no noise!) If you are located in a
city or suburban area and you are restricted in the size of your
antennas and beset by noise then the loop is essential for long/medium
wave boradcast and 160/80 meters. To be used effectively you
require a low noise preamplifier or receiver with a generous AGC
margin. The reciever should be a fully sheilded communications radio
because of the extreme requirement to reject noise.

The sheilded loop antenna , because it is so good,
must preforce come with some caveats ! The main limitation is the upper
frequency limmit. Because they are typically constructed with
coaxial cable, the loop inductance, being proportional to the square of
the diameter, resonates with the distributed capacitance of the cable
at a surprisingly low frequency. A successfull shielded loop is a
compromise between sensitivity which depends on the number of turns,
the band of frequencies to be covered and self resonance. The
loop at resonance has an extremely high Q, and you will discover that
the depth of its power of rejection is strongly related to its Q.

The
loop is also inconvieient because it requires tuning with a variable
capacitor which makes mounting and using the loop cumbersome. For this
practical reason the loop must be used as close as possible to the
reciever.

The loop should be wound with the largest coaxial
cable that you can find. It should have large conductance, that means
50 ohm, RG8 or larger if available. Large diameter Cable TV
hardline would be ideal. It should have a foam dilectric to minimise
distributed capacitance.You will find that the most frustrating
limitation is self resonance, and for that we need 75ohms coax with its
inherently reduced distributed capacitance, but oh bother! 75 Ohm coax
has smaller conductors, limiting the loop Q. We just cannot win.
I have found, in practice, lower distributed capacitance is the
thing to aim for, as this increases the usefull frequency range of your
loop. Problem is "you cant get the wood"! Large diameter 75Ohm
cable is just not available. You may have to compromise and just
use 75Ohm foam dilectric CATV cable like RG6.

In practicc,
the maximum usefull upper frequency is determined by self resonance in
any real system, and this is reached at about 4 Mhz wound with any real
world coax. There is also little to be gained with a loop above
this frequency because the loop can really only null out groundwave
radiation. Ionospheric reflected radiation cannot really be
nulled out because it has low spatial coherence...I mean, it does not
have a clear "point source". Nearly all "usefull" long and medium
wave signals only have a groundwave component, and that means an
interfering transmitter can be eliminated ( as with the noise) which is
great for chasing medium wave dx.

My loop featured in the
pictures has a mean diameter of one meter, low capacitance ,
heavy solid copper armour/shield instrumention cable wound with
only two turns series connected inside the shield is self
resonant at 2.7Mhz. For that result I kick myself because I really
wanted this to work at 80 meters! It is a superb antenna for 160
meters. The lower limit is set by my 1200pf variable capacitor
and that occurs just at 1Mhz.

Performance wise.
At midnight at my QTH in an outer Melbourne suburb the main noise
sources are general lighting, my neighbour's wretched plasma screen
(only 10 meters away too!) , the shack computers and leakage from the
cable TV system. I picked up some "rare dx" on 160 meters, VK2,5
hams on CW, I'm in Melbourne, VK3. This was impossible on the
long wire. Special broadcast stations from VK2 in the 1700Khz
special broadcast allocation were clearly audible and well above
the noise, and completely inaudible on the long wire. Lower down the
band , the 1Khz heterodynes with international medium stations on
10Khz spaced allocations ( Australian medium wave stations are on a
9Khz spaced allocation, hence the distinctive heterodynes)were
audible. Stations in south australia and NSW were clearly
distinguishable and seperable, even on the same frequency by nulling
out the unwanted station. The maximum depth of null on my loop
was somewhat dissapointing, I have built better performing loops in the
past.I think this is due to the central pickup coil not being
shielded, and as you can see from the pictures, loop symmetry is far
from perfect.My other loops have allways had the sheild
symmetrically broken. As this is impractical on this design, the outer
conductor runs continuously around the loop. Some argue about the
neccesity of a symmetic shield, but I think this is still required to
get the ultimate noise rejection and null depth.

Dont forget, the sheild must be split, otherwise its just a shorted turn!

This
loop can be rotated and resonated from the operating position! Its
outdoors, up in the sky. So far my neighbours have been too polite or
shy or apathetic to comment about it. It also repels vampyres!

I
strongly urge radio hams to seriously experiment with the sheilded
loop, it is non critical, easily reproduced and the only real way to
contemplate operation on 160 and 80 meters in a suburban
situation. Note well, its only for RECEPTION. Sheilded Loops make
a profoundly ineffective transmit antenna. Long wires are ok for transmitting, lousy for reception.

cross arms are one meter

pickup coil has 3 turns

13.7 vampyres successfully repelled!

connections to the loop, shield,pickup coil

inductive pickup loop

mounting post on roof

underoof mounting on roof truss

underoof mounting on roof truss

handle on the "armstrong rotator"

The fully rotatable mount consists of
two concentric tubes, the inner can rotate and the conductors are
threaded through this. There is a homemade thrust bearing at the top.
The central pole also mechanically supports the loop.

construction details.The
loop is wound with Hubner and Suhner "Radox" (tm) industrial
instrumention cable. This expensive solid copper armoured and
shielded cable is used in industry for control box cabling that need
armouring and shielding. I found half used discarded drum of this
expensive and excellent cable in a dumpster at work. The solid copper
sheath surrounds two conductors insulated with mica and a high
temperature insulator with a heavy gauge pretinned conductors.
Sorry, you cant have any ! Its very stiff and the solid
copper sheild makes it allmlost self supporting. This stuff will form
the basis of the transmit loop, in the fullness of time. Distributed
capacitance is between that of RG8 and RG6. The conductors have been
joined to make a two turn coil. The loop conductors and pickup coil are
brough down in the inner rotating tube to a variable capacitor mounted
just under the roof.

The fully rotatable mount consists of
two concentric tubes, the inner can rotate and the conductors are
threaded through this. There is a homemade thrust bearing at the top.
The central pole also mechanically supports the loop. The loop
arms are made from 25MM PVC electrical conduit which has just bearly
enough mechanical strength for this job. The central spider is
perspex.

Pre-amp for 160M loop antenna

I
have found that a low noise pre amplifier for 160m reception through
this loop is absolutely essential. The very low efficiency of the loop
antenna is only compensated by its ability to reject noise , which in
the suburban envoironment is extreme. The loop will resolve
signals that are totally inaudible with the main 160m vertical
antenna.

The common base configuration gives this
amplifier an extremely low input impedance, of the order of 5 ohms,
so it makes a particularly good match for the low inductance
pickup coil in the center of the sheilded loop. At this low
frequency just about any modern solicon transistor will do, I
chose a 2n2219A because these have usefull gain up to 150Mhz and their
1 Watt rating means that a sensibly large amount of collector current
may flow, which gives good immunity to broadcast band cross
modulation. The ferrite bead in the collector is required to
prevent VHF oscillation. The presence of parasitic oscillation is
indicated if there is a huge amound of broadband noise and no evidence
of real signals.

I
have used 0.1uF chip capacitors, their low parasitic inductance makes
them ideal bypass and RF coupling capacitors. The collector
inductor was made from a mystery found toroid that exhibited a
usefull Q at 2Mhz and sufficient turns wound to make up 27uH.
I used this method , first
, to ensure that the coil would work as required. The Q is
sufficiently high to require the use of a variable capacitor on the
output to peak tune the whole 160m band. There is
sufficient capacitance reserve to permit tunign down to the top of the
broadcast band to permit hunting for broadcast band DX on the loop.

Images
of the the little preamp. PCB was quickly made with the Dremel Method,
just like the paddy boards, but with less fiddly work. Notice
extensive use of surface mount chip capacitors. These can be
obtained in large numbers from dead consumer durables using a
heat gun to desolder them. They are highly recommended for RF bypassing
service. Capacitors that are meant to be part of resonators should be silver mica.